FIG. 1 shows one example of Fourier transform spectrometer using a rapid scan interferometer typified by a Michelson interferometer. In FIG. 1, a light source 1 emits continuous light having a constant intensity, as shown in FIG. 2(a). The emitted light enters an interferometer 7 composed of a half mirror or beam splitter 2, a fixed mirror 3, and a moving mirror 4. The light is first divided by the mirror 2 into two beams one of which is directed to the fixed mirror 3, while the other is directed to the mirror 4 moving at a constant speed. The first beam is reflected by the fixed mirror 3, passes through the half mirror 2, and travels toward a detector 5. The second beam is reflected by the moving mirror 4 and then reflected toward the detector 5 by the half mirror 2. These two beams interfere with each other while traveling toward the detector 5. As a result, an interferogram as shown in FIG. 2(b) is obtained from the detector 5. A sample 6 is placed, for example, between the interferometer and the detector 5. The interferogram is sampled and converted into digital form by an analog-to-digital converter 8 in response to clock pulses (FIG. 2(c)) produced at intervals of t by the interferometer. The resulting interferogram (FIG. 2(d)) is sent to a computer 9, which takes the Fourier transform of the interferogram. As a result, a spectrum is derived.
The Fourier transform spectrometer of this construction uses continuous light of a constant intensity as shown in FIG. 2(a) and, therefore, limitations have been imposed on usable light sources. Also, only those samples which are not affected by continuous illumination of light can be investigated by this apparatus.